Sound reproducing system



Sept. 15, 1936. s, BALLANTINE 2,054,647

SOUND REPRODUC ING SYSTEM Filed Nov. 21, 1953 3 Sheets-Sheet l lawpassHY/er luv Sound Pressure F'requency, oic/Y/af/bns oer Jecond :jmwnto o',

Sept. 15, 1936. s. BALLANTINE YSQUND REPRODUCING SYSTEM Filed Nov. 21,1955 3 Sheets-She'c 2' P 1936- s. BALLANTINE v 2,054,647

SOUND REPRODUCING SYSTEM Filed Nov. 21, 1935 3 Sheets-Sheet 3 Fl/ferMuff n Patented Sept. 15, 1936 PATENT OFFICE SOUND REPRODUCING SYSTEMStuart Ballantine, Mo

signor, by mesne assi ration of America, ration of Delaware untain-Lakes, N. J., as gnments, to Radio Corpo- New York, N. Y., a corpo-Application November 21, 1933, Serial No. 699,069

' 4 Claims.

This invention relates to sound reproducing systems and moreparticularly to electro-acoustic sound reproducing systems-such as areemployed in radio broadcast receivers.

The invention is particularly useful in connection with radio receiverswhere conditions of noise on weak signals and other kinds ofinterference make it desirable to restrict the range of reproduction atthe high frequency end, but it is also useful in general with any kindof electroacoustic system in which it is desirable to adjust therelative amplitudes of the high and low frequenoies.

An object of the invention is to provide a sound reproducing systemincluding electrical elements for regulating the tone quality of thereproduced sound. An object is to provide a sound reproducing system ofthe electro-acoustic type and including adjustable elements forregulating the intensities of the higher frequency components withrespect to the lower frequency components. Another object is to provideelectrical sound'reproducing, systems which include simple circuitelements for restricting at will the upper limit of the frequency rangeof reproduction. A further object is to provide a radio receiverincluding circuit elements for automatically restricting the audio rangeof reproduction as the antenna signal voltage decreases.

These and other objects and the advantages of the invention will beapparent from the following specification when taken with theaccompanying drawings, in which:

Figs. 1 and 2 are circuit diagrams of sound reproducing systems whichinclude adjustable elements for effecting a tone control;

Fig. 3 is a curve sheet illustrating the relation of sound pressure tofrequency for different adjustments of systems such as shown in Figs. 1and 2;

Figs. 4 and 5 are circuit diagrams of other types of tone controlsystems;

Figs. 6 and 7 are circuit diagrams of radio receivers which includeelements for automatically varying the range of audio reproduction inaccordance with the strength of the received carrier wave signals;

Fig. 8 is a fragmentary circuit diagram illustrating a modification ofthe Fig. 7 circuit arrangement; and

Fig. 9 is a circuit diagram of a radio receiver including a differentsystem for automatically controlling the range of audio reproduction asa function of the received signal strength.

In the several circuit diagrams, the sound re-" producing apparatus isshownas a composite system including a loud speaker I for thereproduction of the lower frequencies and a loud speaker 2 for thereproduction of the higher frequencies. The speakers may be directlyconnected in parallel across a source of audio frequency current, suchas the audio frequency transformer 3 in the output circuit of vacuumtube 4. If desired, sharp cut-ofi characteristics may be obtained withspeakers which lack such characteristics by supplying power to thespeakers I, 2 through a low-pass filter 5 and a highpass filter 6,respectively.

With either system, a tone control adjustment is provided by the switchI which may be opened to interrupt the supply of energy to the highfrequency speaker 2. If the opening of the switch 1 produces anundesirably large change in the impedance characteristics of thecircuit, a compensating network may be connected across the audiocircuit when the speaker 2 is removed. As shown in Fig. 2, the switch Iconnects a resistance 8 across the high pass filter when the circuitthrough the speaker 2 is opened, the resistance 8 approximating theimpedance of' the reproducer 2 and being of a proper value to terminatethe high frequency filter without reflection losses.

In the case of a radio receiver, the switch 1 may be mounted on thecontrol panel to provide a simple and inexpensive means for restrictingthe range of reproduction. This method of tone control has substantialadvantages over methods involving the insertion of networks or filtersin the audio amplifier or the radio frequency circuits.

The effect obtained by this tone control will be apparent from aconsideration of the sound pressure curves of Fig. 3. The solid linecurve AB represents the sound pressure response of the compositereproducer system and it will be noted that the response or audio outputis substantially uniform for all frequencies between about 50 and 10,000cycles when both reproducers are used. The curve AAA" represents thecontribution of the low frequency speaker I whilecurve B'Bf'B representsthe contribution of the high frequency reproducer 2. When the switch Iis opened, the higher frequencies are not reproduced and the system hasa relatively sharp cut-01f at about 4,000 cycles.

It is to be understood that the illustrated location of the switch 1 ispurelydiagrammatic as the switch may be located in any appropriatecircuit where it may be operated to render the reproducer 2 inoperative.For example, in the case of a coil-driven device employing anelectromagnet, the exciting current to the electromagnet may beinterrupted and, in the case of an electrostatic reproducer, thepolarizing voltage may be removed.

In place of suppressing all audio output above a certain limit, it maybe desirable in some cases merely to attenuate the higher frequencies.An appropriate arrangement for this method of tone control is shown inFig. 4 as an adjustable attenuator 9 in the form of a T-type constantresistance device connected in the supply circuit to the high frequencyreproducer 2, or simpler types of attenuators such as a variable seriesresistance or inductance, or a variable shunt capacity may be employed.The remaining circuit elements of the reproducer system may besubstantially as illustrated in Fig. 2.

As illustrated in Fig. 5, a control of the relative intensities of theaudio energy supplied to the repro-ducers l, 2 may be obtained byproviding separate high and low frequency channels for the reproducers.One or both channels include a transmission control element, such as thevoltage dividers lil, l l and each channel may include an amplifier I2.In the operation of systems of this type, it is usually sufiicient toset the output of the low frequency reproducer l at some predeterminedlevel by adjustment of the control l0, and to obtain an adequate tonecontrol solely by adjustment of the transmission in the high frequencychannel.

In a radio receiver, for example, it is usually desirable to place onlyvoltage divider II on the control panel and to place the voltage dividerID at some point on the chassis which may be conveniently reached fromtime to time, if desired, for effecting a relatively permanent orfactory adjustment of the low frequency transmission.

This method of tone control by variation of the transmission efiiciencyof separate high and low frequency channels is especially adapted forthe automatic control of the tone quality of reproduction as a functionof the received signal energy. In ordinary circumstances, the receptionof weak radio signals is accompanied by noise and undesirabledistubances of many kinds which are, in general, of relatively highfrequency. On the other hand, strong radio signals are not usuallyaccompanied by such disturbing noises. It is therefore desirable, ingeneral, to restrict the audio range of reproduction when receiving weaksignals and to employ the full range of reproduction when strong signalsare received. Radio receivers which include circuit elements forautomatically controlling the range of reproduction as a function of thereceived signal energy are illustrated in Figs. 6 to 9, inclusive.

As shown schematically in Fig. 6, the radio receiver includes a radioamplifier 13 of any desired type, and a detector and/or audio amplifierI4 across which the low pass filter 5 and the high pass filter 6 areconnected in parallel. An audio amplifier I2 is provided between the lowpass filter 5 and the reproducer l and, if desired, a transmissioncontrol such as the voltage divider I0 of the Fig. 5 circuit may beemployed but, in general, such adjustable control is not necessary. Thehigh frequency channel includes an amplifier tube l5 which is associatedwith an automatic gain control system, and may include a furtheramplifier stage I2 of the constant gain type. The automatic controlsystem includes the rectifier l6 which is connected across desiredpoints in the radio frequency system, for example in parallel with thedetector I4, the rectifier being preferably of the diode type andsubjected to a bias voltage from a current source, indicateddiagrammatically as a battery ll, which prevents the development of arectified output so long as the received signal energy is less than apredetermined critical value. The rectified voltage is developed acrossan output resistance. I8 which is shunted by a radio lay-pass condenserl9, and preferably, this direct current voltage is employed. in theknown manner to effect an automatic control of the detector inputvoltage. The direct current component of the rectifier system is appliedto the control grids of radio amplifier 13 through the lead 20, thealternating current components of modulation frequencies being removedby the filter comprising resistance 2| and capacity 22.

In accordance with this invention, the direct current output of therectifier system is employed to effect an automatic control of thetransmission efiiciency of the high frequency channel. The filtereddirect current voltage developed by the rectifier system is impressedupon the grid of a tube 23 which has a resistor 24 connected between itscathode and the plate current supply 25. The voltage drop producedacross the resistor 24 by plate current flow in tube 23 is applied as anegative bias to the control grid of the amplifier tube 15 through aresistor 26. In the absence of an antenna input, the tube 23 is sobiased by a current source 21 that a relatively heavy plate current flowis established and the voltage developed across resistor 24 issufiicient to reduce the transmission through the amplifier tube I5 to anegligible value. As the antenna voltage increases, the rectifier systembecomes operative and the direct current voltage developed across therectifier output resistance l8 makes the bias voltage on tube 23 morenegative, thus reducing the plate current flow and therefore the directcurrent voltage which is developed across the resistor 24. As thevoltage across the resistor 24 decreases, the negative bias on theamplifier 15 decreases and the transmission through the high frequencychannel is correspondingly increased.

The tube l5 may be of the tetrode type and a resistance 28 is preferablyshunted across the output transformer 29 of the tube to maintain aconstant output impedance for varying grid bias voltage.

The described circuit effects an automatic decrease in the transmissionof the higher frequencies as the antenna input decreases. Various ratesof control and of the relation between transmission and antenna voltagesmay be obtained by suitable choice of the tubes and energizingpotentials applied thereto, as will be apparent to those familiar withthe design and construction of radio circuits. I have found thatadjustment particularly useful which provides a constant normaltransmission through tube I5 for all signal voltages exceeding a certainvalue and which gradually decreases the transmission as the signalvoltage falls below that predetermined value.

While the circuit of Fig. 6 employs a rectifier system for automaticallycontrolling the ampllfier gain and the range of audio reproduction, itwill be apparent that the automatic gain control system could be omittedwithout affecting the operation of the tone control system or,alternatively, the tube 23 could be energized by the potential dropacross a resistor in a volume control circuit of the manually regulatedtype. Whether the variation of volume control voltage is developedautomatically or is adjusted manually, the same automatic regulation oftone control with varying antenna input will be obtained.

Another method of automatic tone control isillustrated in Fig. 7. Aconventional type of radio amplifier l3 works into a detector I4 whichis followed by an audio amplifier. 30. The radio amplifier tubes areinitially biased for high gainby appropriate means, such as a currentsource 3| and means is provided for automatically regulating the gain ofthe amplifier to maintain an approximately constant input to thedetector M.

A gain control system similar to that of Fig. 6 is illustrated, theseveral parts being identified by corresponding reference numerals, butit is to be understood that the exact design of the radio amplifier andthe automatic gain control system is not an essential part of theinvention.

The low frequency reproducer I and the high frequency reproducer 2 areconnected in parallel across the audio amplifier 30 and, in accordancewith the invention, a switch 1 in an energizing circuit of the highfrequency speaker 2 is automatically controlled as a function of thereceiver input or of the adjustment of the radio amplification. Theswitch 1 forms part of a relay or electromagnetic switch in the platecircuits of the controlled tubes of the radio amplifier l3. Asillustrated, the plate circuits of the controlled tubes are returned tothe plate supply source 32 through a tuning meter 33 and the winding 34of the relay or solenoid which controls the switch I. The winding may beshunted by an adjustable resistance 35 for regulating that value ofplate current which is suificient to attract the armature to open theswitch 1. An additional contact or switch 35 may be provided, ifdesired, to control the circuit of a signal light 31 for indicatingwhether the composite reproducer system or only the low frequencyreproducer is in operation. The determination of the critical signalstrength at which the tone control switch is to be operated is a matterof judgment and experiment, and will doubtless depend upon the noiseconditions existing at the particular receiver location. When the signalstrength falls below the predetermined critical value, the increasedplate current flow in the radio amplifier results in the attraction ofthe armature of the control relay to open switch I, thus deenergizingthe high frequency reproducer 2.

Another type of control is illustrated in the fragmentary circuitdiagram, Fig. 8, which includes only those elements which follow a radioamplifier of the automatic gain control type such as, for example, theelements shown to the left of the broken line in Fig. '7. The audiosystem following a detector 4 and audio amplifier 30 provides but onechannel for the transmission of audio frequency energy to thereproducers but a low pass filter 5 is automatically switched into thetransmission channel when the received signal energy falls below apredetermined critical value. The relay elements 34, 35 may be the sameas shown in Fig. 7, but double contacts 38, 39 are provided to connectthe low pass filter into the audio circuit when the plate current flowrises above a critical value and to by-pass the audio energy around thefilter when the plate current flow falls below the critical value. The

filter 5' has an upper cut-ofi adaptedto attenuate the higherfrequencies to a desired extent when the filter is inserted in the:audio system. For greater clarity of illustration, the reproducer systemis shown as including. the separate speakers I, 2 but it will beapparent that a single speaker may be used if it is capable ofreproducing the full range of audio frequencies.

Other circuit arrangements maybe employed for-obtaining automatic toneregulation by the methods illustrated in Figs.v 7 and 8. The systemshown in Fig; 9 is generally like that of Fig. 7 but employs a separatetube 40 to control therelay switch system 34, 35, 1', in place of theplate current of the controlled tubes of the radio amplifier It. Likeelements of. the circuits of Figs. 7 and 9 are identified bycorresponding reference numerals, but will not be described in detail.The relay winding 34 is included in the plate circuit of the tube 40,and the bias on tube 40 is automatically varied asa function of thereceived signal energy by connecting the grid to the automatic bias lead20 of the gain control system. The desired quiescent bias on tube 40 maybe so adjusted, by a current source 4|, that plate current of tube 40energizes relay winding 35 to hold switch 1' open until the addednegative bias component due to rectification by tube I6 reaches a valuecorresponding to that critical receiver input for which full rangereproduction is desired. This circuit is of particular advantage inpermitting precise operation of the relay in the region of weak signals.This result is accomplished by employing a tube 40 which has arelatively sharp plate-current cut-01f rather than, for example, one ofthe variable-mu variety having a gradual cut-01f, such as would beemployed in the radio amplifier l3. The plate current of tube 48 willthen remain at zero over a considerable range of signal strength in theregion where the latter is weak, and this action permits more preciseoperation of the relay at the critical signal strength for which it isadjusted.

It will be apparent that the described circuits are illustrative of thewide range of appropriate constructions for obtaining a simple andeffective tone control. Many variations will be obvious to thosefamiliar with the design of vacuum tube circuits. For example, relaycontrol systems such as shown in Figs. 7 and 8 may be employed in radioreceivers in which the sensitivity is controlled manually by adjustmentof the control grid bias on the radio amplifier tubes.

These and other variations or modifications in the current arrangementsfall within the spirit of my invention as set forth in the followingclaims.

I claim:

1. In a radio receiver, the combination with a radio amplifier, adetector, and an audio system including an audio amplifier and soundreproducing means, of means for automatically controlling the radioamplifier gain to maintain an approximately constant radio input to saiddetector over a Wide range in the magnitude of the radio input to saidreceiver, and means for controlling the width of the band of audiofrequencies which is reproduced by said sound reproducing means; thesaid gain controlling means and band width controlling means including,in common, rectifier means for developing a direct current voltage whichvaries with the radio input to said receiver, said band widthcontrolling means including a direct current amplifier, and meansenergized by the output of said direct current amplifier when saiddirect current voltage exceeds a critical value for providing the solebiasing of the grid of the audio amplifier in a manner such thattransmission by said audio system of the higher frequency end of saidband is blocked.

2. In a sound-reproducing system, the combination with a detector, ahigh frequency and a low frequency sound reproducer, a separate highfrequency and low frequency amplifier between said detector and therespective sound reproducers; of means for blocking reproduction by saidhigh frequency reproducer, and means controlled by the radio voltageinput to said detector for actuating said blocking means, said blockingmeans including means for rendering said high amplifier inoperative byproviding the sole negative bias therefor.

3. In a sound-reproducing system, the combination with a detector, of ahigh frequency and a low frequency reproducer, a separate audiotransmission channel between said detector and each of said reproducers,and an amplifier coilpled to the high frequency reproducer, means solelybiasing the amplifier so that for input signals below a predeterminedlevel the amplifier is blocked, and additional means arranged to controlsaid biasing means so that for input signals above said level theamplifier is operative to transmit signals to the reproducer coupledthereto.

4. In a sound-reproducing system, the combination with a detector, of ahigh frequency and a low frequency reproducer, a separate audiotransmission channel between said detector and each of said reproducers,an amplifier coupled to the high frequency reproducer, and meanscooperating with the detector for providing the sole negative bias onsaid amplifier, said means automatically controlling the bias on thegrid of said amplifier to vary the bias inversely with variations insignal intensity.

STUART BALLANTINE.

